1. Field of the Invention
The disclosure relates to an interference objective lens and a reference surface unit set to be used for a microscope and an image measurement device or a fine shape measurement device based on the microscope.
2. Description of the Related Art
In fields of measuring a surface texture of a metal surface to be processed and a shape and a step of a circuit pattern of a semiconductor integrated circuit and measuring a thickness of a transparent thin film, an optical macro magnification observation device of a low magnification or an optical microscope having a higher magnification than the device is provided with an interference function and an observed interference fringe is subjected to image processing, so that a fine surface shape is measured with high precision.
In the magnification optical device capable of performing the interference measurement, a member or structure configuring an interference system has an objective lens integrally formed thereto and functions as an interference system, as a single member. The objective lens is referred to as an interference objective lens. The general interference objective lens has an objective lens, a reference surface and a beam splitter. The objective lens is configured to converge light emitted from a light source toward a measurement target. The light converged by the objective lens is split at the beam splitter into a reference optical path facing toward the reference surface becoming a reference and a measurement optical path facing toward a measurement target surface (surface to be measured). The light having passed the reference optical path is reflected on the reference surface, and the light having passed the measurement optical path is reflected on the surface to be measured. The reflected lights are again synthesized at the beam splitter, which is then output as an interference light.
As the light source for causing the interference, white light that is to be generated by an incandescent lamp, an LED or the like, is usually used. Since the white light has low interference, the interference intensity becomes a peak in a narrow range within which an optical path difference between the reference optical path and the measurement optical path is close to zero (0). For this reason, when the interference objective lens is scanned in a height direction (optical axis direction) (a measurement optical path length is changed), heights at which the brightness of the interference fringe becomes a peak are respectively detected at each pixel position in a viewing field of an imaging device, so that it is possible to precisely measure a three-dimensional shape and the like of the measurement target.
Regarding the interference objective lens, a Michelson type and a Mirau type are generally used. The Michelson type is a structure having a reference surface on an optical axis separately provided from a measurement target, and is mainly used for an objective lens of low magnification of 5 magnifications or less. The Mirau type is a structure having a reference surface on the same optical axis as a measurement target, and is used for an objective lens of high magnification incapable of taking a long working distance, as compared to the Michelson type.
JP-A-2005-538359, JP-A-2007-536539 and JP-A-H05-118831 disclose the interference microscope and the interference objective lens of the related art, and Patent JP-A-2000-193891 discloses an adjusting mechanism.
In the case of the microscope objective lens of relatively low magnification, since the numerical aperture (NA) is small, the light is incident on a surface to be measured at a small inclination relative to an optical axis and a working distance becomes longer. Therefore, the microscope objective lens is likely to be influenced by the reflectivity inherent to the surface to be measured having diverse reflectivity, such as a metal surface and a glass surface. For example, upon the actual measurement, when a measurement target having a surface texture of which reflectivity is low, such as glass, is observed by using an interference objective lens having a reference surface with which a contrast of the optimal interference fringe is obtained on a metal reflective surface having high reflectivity, the contrast of a white interference fringe is lowered.
In order to solve the above problem, it is considered to appropriately select and use a reference surface having an optimal reflectivity corresponding to the reflectivity of the measurement target.
However, the white light that is to be used for the interference measurement is incoherent light having a low interference possibility. Therefore, in order to generate the interference, it is necessary to strictly match an optical path length of the measurement optical path on which the measurement target is to be arranged and an optical path length of the reference optical path on which the reference surface is to be arranged. For example, a range in which a clear interference fringe is observed in an optical axis direction is usually a very narrow range of 1 μm or less, and when the optical path lengths are not strictly matched, the interference fringe cannot be obtained as an image. For this reason, a strict positional relation among the split synthesis means, the reference surface and the surface to be measured is required. Also, since the surface to be measured is a focal position of the objective lens, the great difficulty is caused upon manufacturing of a component and during an assembling process. Due to the situations, the interference objective lens is generally shipped with being adjusted and fixed so that the white interference fringe occurs at an ideal focal position of the lens.
Therefore, it is actually difficult to arbitrarily select the reference surface of the interference objective lens in accordance with the measurement target, and it is necessary to prepare a plurality of interference objective lenses each of which has a reference surface corresponding to the reflectivity of the measurement target. Also, when the interference objective lens is integrally provided with the measurement device, it is necessary to prepare a plurality of measurement devices.